Bayer matrix – 25% of total sensor receives red light – the remaining sensor elements are interpolated to form a complete set of RGB values for each – Image from Wikipedia.com

In preparation for the 2017 US Solar Eclipse, I wanted to get in as much scope time as possible, so when the eclipse is upon us, I will be able to reproduce what I have taken in practice. I was interested to see the results of a monochrome camera, versus color – where one has to contend with a CFA (color filter array) known as a Bayer filter. A CFA has to interpolate the received light, per sensor element (pixel), in a demosaicing process, to form a complete set of values per pixel. The SolarMax Hydrogen Alpha etalon filters (for my particular model) give a .5 angstrom bandwidth of red light in the 6562.8 angstrom (656.28nm) wavelength. A standard DSLR or other color CMOS camera, employing a Bayer filter, is only sensitive to 25% of the total light received, for this particular wavelength. With a monochrome sensor, it captures all light at each pixel, regardless of color. Each sensor element on a monochrome sensor receives much more light than a color sensor. As a consequence, image noise is lower – at equivalent ISO speeds, and resolution is higher.

For this attempt, the setup used was the aforementioned SolarMax scope on a iOptron CEM25 center-balanced equatorial mount. Once I was able to level the mount, adjust the counterweight, point it in the general area of Polaris, ensure the latitude adjustment was set to my latitude, verify date and time and check that the GPS had found the correction location, I was ready to point it at the Sun. Next, I did a GOTO selection slew of the mount, from the zero position, to the Sun, and the mount found its way to the general area of the Sun. I then adjusted RA and DEC motors and synced the mount to it. Without any real polar alignment, the mount does exceptionally well for tracking the Sun.

The image shown is without a Barlow lens, or any eyepiece projection, and just with the QHY camera in the 15mm blocking filter diagonal. The intent here was to capture the entire sphere of the sun. The difficult part of this type of shot is dealing with focus. You have air turbulence, some level of wind – usually, and bright sunlight on your computer screen with which to contend. Also, if you don’t have a motorized focuser installed on the scope, adjusting your helical focuser by hand will shake the image. My next project is to look at installing a motorized focuser on the SolarMax OTA, so I can eliminate one aspect of shaking – and aid in fine focus adjustments. Finding an FWHM-like adjustment tool would be handy in this situation. Sharpcap has one, but it’s more for single point star focusing, whereas the sample area of the sun is too large, obviously, and will not allow for FWHM. Air turbulence? Well, that’s out of my control. Until then, I tend to just go back and forth through focus, to land on the sharpest detail I can attain.

Coronado BF15 – 15mm diagonal secondary blocking filter

Next was adjustments of my etalon filters on the SolarMax scope to find some good detail of the Sun. I found that if I crank all adjustments on the scope (Tmax tilt wheel, RichView™ tuning ring and Richview™ tuning lever for the internal etalon) fully counter-clockwise, I get the most detail from the surface of the Sun. Any other combination I tried didn’t seem to give the same amount of detail. Other adjustment options are seemingly for obtaining prominence detail.

Lastly, for acquiring a reasonable set of frames, I tinkered with SharpCap 2.9 settings (resolution, color space, exposure, gain, offset, and most importantly – frame rate – to help compensate for any air turbulence). For this particular image, I captured 50 frames at 2048 x 2048, which comes out to about a 209MB .AVI file. On other grabs I did, I had files in the 3-4GB range, for 200 frames. RegiStax can usually handle such a large file, but for some reason, I could not get it to cooperate with all files. Post processing of those large files would usually fail at the alignment check.

The next section describes my post-processing of the final image, to stack the original 50 frames from the .AVI I created with the camera software, align those frames, and do a noise reduction and sharpening to the data. This took much trial and effort to get a decent result.

Solar telescope field setup, with small tent to shield laptop from sun. Power unit is a Goal Zero Yeti 400

Image Processing

With the RegiStax software, I was able to pull in the .AVI file, select my alignment points (a lower value appears to work better), and press the ‘Align’ button. The result is a bunch of red dots (click on the image for a better view), which is coincidentally the amount of alignment points you selected from the alignment points slider.

From the result of the alignment, there is a frame selector slider bar at the base of the images. From this you want to select the frame which has the least amount of shift (as indicated by the lines extending from the circle – click image for better view). This frame has some shift to correct.

The frame below has less shift, as seen there are few lines extending from the alignment points. I chose this frame from which to base my stack.

Stacked image below, after alignment.

Next, was to run this through a denoise and sharpen adjustment. Below is the image from the stacking process that I just completed.

Last, was to apply a pre-built denoise and sharpen scheme to the image. Below is the result.

After saving the image from Registax, I ran it through Photoshop to adjust curves, levels, Smart Sharpen, etc… and gave it some color by enabling the Hue option and adjusting saturation. Final image below.

This was my first run at this camera, and I’m happy with the results. The monochrome camera definitely gave me more detail than my color imager. Unfortunately, the surface detail during this exposure wasn’t as active as I was hoping for, only a few spots were seen and I can’t make out any filaments. As mentioned, I hope to install a motorized focuser and find a software tool to help with fine focus. I believe that will help me over the hurdle to obtain the sharper results I’m after. Thanks for reading.

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